Handling of lines to scroll setting on win32

359df028 introduced multiplying smooth scroll deltas by SPI_GETWHEELSCROLL_LINES on win32. While this makes actual scrolling of widgets contained in a GtkScrolledWindow behave as intended by the platform, this also affects widgets that use the mouse wheel for zooming as in the zoom factor per scroll wheel click now depends on the 'how many lines to scroll' settings. This is clearly not intended since a non-exhaustive investigation found no other win32 apps change their zoom increments based on that setting.

Suggested solution:

Don't multiply in the win32 backend

Add a method to gdk that finds out how many lines to scroll per scroll wheel click. Most likely, this method will just return 1 on all backends but win32.